I am a huge fan of Elixir and Phoenix even without OTP, don't get me wrong. It's my go-to framework now, replacing Rails. I just ended up hitting the OTP chapters in Programming Elixir and Programming Phoenix, and just kind of wondered why I was doing what I was doing, and why anyone would ever have or want to do this.

That's been covered pretty well by other commenters, but it seems like it's something you have to experience yourself in order to really understand.